I think North at fault here, to be honest. The 2 bid (a reverse by Responder) is game-forcing in almost any system. That being the case, we should apply the principle of "Fast Arrival" to the 3 bid by South, ie: 3 is stronger and more encouraging (ie: slam-invitational) than 4 would be. That being the case, I feel North should be doing something other than timidly signing off in 4 .

At the end of the day what action that is depends largely on your cue-bidding style, but 3 or 4 must be cue-bids for Hearts (4 perhaps more unambiguous). Now when South doesn't show a Diamond Control, the value of North's Diamond singleton becomes apparent and North should have no further difficulty in pushing to the slam. So...

1 - 1
1NT - 2
3 - 4 // Cue
4 - 4NT // Spade Cue, no Diamond Control / RKCB
5 - 6
All Pass

This.
Though I prefer the 3 control, rather than 4 .

Also, while the 2 bid is almost universally played as a game-force, it does not necessarily promise a five card suit. In other words . . . "Fast Arrival" does not apply. The 2 rebid could simply be a means to force game while keeping the bidding low.

As an example, picture responder with this hand: KQ AKJT 2 KT9876 . . . and, he still goes through the 2 GF bid on the way to exploring slam in s.